Classification of Industrial Robots
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Based on the form of operational coordinates, they can be categorized as follows:
(1) Cartesian Industrial Robots
Their motion consists of three mutually perpendicular linear movements (i.e., PPP), forming a rectangular working space. The movement distances in each axis can be directly read out, making programming calculations for positions and orientations intuitive. They have high positioning accuracy, uncoupled control, and a simple structure, but they occupy a large volume and have a small range of motion, making them less flexible and difficult to coordinate with other industrial robots.
(2) Cylindrical Industrial Robots
Their motion is achieved through a system consisting of one rotation and two linear movements, creating a cylindrical working space. Compared to Cartesian industrial robots, they occupy less volume while having a larger range of motion under the same working space conditions. Their positioning accuracy is only slightly less than that of Cartesian robots, but they are also difficult to coordinate with other industrial robots.
(3) Spherical Industrial Robots
Also known as polar industrial robots, their arm movements consist of two rotations and one linear movement (i.e., RRP, one rotation, one pitch, and one extension movement), creating a spherical working space. They can perform pitch movements and grasp coordinated workpieces from the ground or lower positions. Their positioning accuracy is high, and the positioning error is proportional to the arm length.
(4) Articulated Industrial Robots
Also known as revolute coordinate industrial robots, these robots have arms similar to a human upper limb. Their first three joints are revolute pairs (i.e., RRR). This type of industrial robot typically consists of a column and upper and lower arms, with the column forming a shoulder joint with the upper arm and the elbow joint with the lower arm. This allows the upper arm to rotate and pitch while the lower arm can pitch up and down. They have the most compact structure and high flexibility, occupying the smallest footprint, and can coordinate with other industrial robots, but they have lower positioning accuracy, balance issues, and coupled control. This type of industrial robot is increasingly widely used.
(5) Planar Joint Industrial Robots
They use one moving joint and two revolute joints (i.e., PRR), with the moving joint allowing up and down movement, while the two revolute joints control forward, backward, left, and right movements. This type of industrial robot is also known as SCARA (Selective Compliance Assembly Robot Arm). It has compliance in the horizontal direction and high rigidity in the vertical direction. Its structure is simple, and movements are flexible, making it widely used in assembly operations, especially suitable for the assembly of small-sized components, such as in the electronics industry.
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Based on the drive method, they can be categorized as follows:
(1) Pneumatic Industrial Robots
This type of industrial robot is driven by compressed air. Its advantages include a convenient air source, rapid movements, simple structure, low cost, and no pollution. However, the compressibility of air leads to poor stability in working speed. Additionally, since the air source pressure is generally only around 6 kPa, these industrial robots have a low lifting capacity, typically only a few dozen Newtons, with a maximum of over a hundred Newtons.
(2) Hydraulic Industrial Robots
Hydraulic pressure is much higher than pneumatic pressure, generally around 70 kPa, so hydraulic-driven industrial robots have a greater lifting capacity, reaching thousands of Newtons. They have compact structures, smooth transmission, and quick movements, but they require high sealing standards and are not suitable for high or low-temperature environments.
(3) Electric Industrial Robots
This is currently the most widely used type of industrial robot, not only because there are many varieties of electric motors providing various options for industrial robot design but also because they can utilize multiple flexible control methods. Early models primarily used stepper motors, later evolving to DC servo drive units, and now AC servo drive units are also rapidly developing. These drive units either directly drive the operational mechanism or reduce speed through devices like harmonic reducers, resulting in very compact and simple structures.
Industrial Robot Control Systems
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Characteristics of Industrial Robots
The control technology of industrial robots has developed based on traditional mechanical system control technology, so there is no fundamental difference between the two. However, industrial robot control systems also have many unique aspects. Their characteristics are as follows:
(1) Industrial robots have several joints, with typical industrial robots having five to six joints, each controlled by a servo system, requiring coordination among multiple servo systems for joint movements.
(2) The working tasks of industrial robots require the hand of the operational mechanism to perform spatial point movements or continuous trajectory movements. The motion control of industrial robots necessitates complex coordinate transformation calculations and inverse matrix function operations.
(3) The mathematical model of industrial robots is a complex model that is multivariable, nonlinear, and has variable parameters, with coupling existing among various variables. Therefore, complex control techniques such as feedforward, compensation, decoupling, and adaptive control are often used in industrial robot control.
(4) More advanced industrial robots require measurement and analysis of environmental conditions and control instructions, using computers to establish a large information database and employing artificial intelligence methods for control, decision-making, management, and operation, automatically selecting the best control rules according to given requirements.
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Basic Requirements for Industrial Robot Control Systems
(1) To achieve control functions for the position, speed, and acceleration of industrial robots, and for those industrial robots performing continuous trajectory movements, they must also possess trajectory planning and control functions.
(2) Convenient human-machine interaction functions, allowing operators to use direct command codes to instruct industrial robots. The use of industrial robots should have memory, correction, and program jumping functions for operational knowledge.
(3) The ability to detect and sense external environments (including working conditions). To enable industrial robots to adapt to changes in external states, they should be capable of measuring, recognizing, judging, and understanding information related to vision, force, touch, etc. In automated production lines, industrial robots must communicate with other devices and coordinate work.
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Classification of Industrial Robot Control Systems
Industrial robot control systems can be classified from different perspectives. For instance, based on the method of controlling motion, they can be divided into joint control, Cartesian space motion control, and adaptive control; based on trajectory control methods, they can be divided into point control and continuous trajectory control; and based on speed control methods, they can be divided into speed control, acceleration control, and force control.
(1) Program Control Systems: Applying a certain regular control effect to each degree of freedom allows the robot to achieve the required spatial trajectory.
(2) Adaptive Control Systems: When external conditions change, to ensure the required quality or to improve control quality as experience accumulates, the process is based on observing the state of the operational mechanism and servo errors, adjusting the parameters of the nonlinear model until the error disappears. The structure and parameters of such systems can change automatically over time and conditions.
(3) Artificial Intelligence Systems: These systems do not pre-compile motion programs but require real-time determination of control actions based on surrounding state information obtained during motion. When external conditions change, to ensure the required quality or to improve control quality as experience accumulates, the process is based on observing the state of the operational mechanism and servo errors, adjusting the parameters of the nonlinear model until the error disappears. Therefore, this system is a type of adaptive control system.
